1. Field of the Invention
This invention relates to an interrogation probe circuit in a vehicular collision avoidance system, more particularly, of the SECANT type.
2. Description of the Prior Art
Collision avoidance systems have been proposed as means of preventing collisions by vehicles both of the maritime vehicle type and of the airborne type. Cooperative systems of the asynchronous type include interrogator-transponder techniques in which interrogation signals or probes are transmitted from one vehicle on a random basis. Vehicles in the vicinity receiving such probes respond with signals of various indicia to provide to the interrogation vehicle information relating to both range and information identifying the vehicle and its position. In airborne systems such information may include the relative or actual altitude of the aircraft.
An existing collision avoidance system known by the acronym SECANT (Separation Control of Aircraft by NonSynchronous Techniques) employs probes identified by any one of a plurality of frequencies and replies using different frequencies of the same band but arranged into a predetermined correspondence to a particular probe frequency. Special correlation techniques separate the true reply received by any one vehicle from received reply signals induced by probes from remote vehicles, the latter replies being generally identified as "fruit." Such a system is described in U.S. Pat. Nos. 3,755,811 issued Aug. 28, 1973, and 3,803,608 issued Apr. 9, 1974, based on the inventions of Jack Breckman, as well as U.S. Pat. No. 3,803,604 describing a "Digital Tracker," issued Apr. 9, 1974, to Bernard Case. A correlator for such a system is described in U.S. Pat. No. 3,887,916 issued to R. B. Goyer on June 3, 1975.
In such systems singular or multi-path reflections of the probe signals either from the surface of the vehicle itself or from reflections from the terrain can be received by the prober's antenna system and cause false alarms. Such false alarms can disrupt the orderly processing of the correlator and tracker thereby masking or dominating the true replies with the false replies that are, indeed, worse then the typical "fruit" replies from other vehicles. Such autogenetic false alarm signals can cause a vehicle to track itself and disrupt the orderly operation of the system.